Generally, the present invention relates to fiber optic communication systems, and particularly to combiners for combining laser beams from different lasers, to produce a depolarized output.
In many optical systems, such as pumps in optical communication systems, fiber gyros, control channels in optical amplifiers, sensors, and inteferometers, it is desirable to have a depolarized source to reduce polarization effects. For example, optical communications systems are increasingly using fiber systems that include fiber amplifiers for amplifying the optical communications signal. In particular, Raman amplifiers are becoming increasingly used for distributed or remote amplification. These amplifiers are useful because they may be implemented in the fiber that carries the optical communications signal, and do not require the insertion of a special type of fiber, as is the case with a rare earth-doped fiber amplifier. As a result, systems may be designed where amplification occurs in the transmission link itself, allowing greater transmission distances between amplifiers, higher receiver sensitivities, and lower transmission powers.
Furthermore, the wavelength range over which a Raman amplifier manifests gain is determined by the wavelength of the pump light. Therefore, a number of pump lasers at different wavelengths may be used to provide gain over a wide wavelength range. This contrasts with the rare earth-doped fiber amplifier whose gain bandwidth is determined by the rare earth species doped in the fiber. Raman amplification is also a very low noise process, limited primarily at low powers by pump noise and quantum noise effects.
One particular concern with Raman amplification is the polarization dependence of the amplification process. The Raman gain coefficient for pump light polarized parallel to a linear signal polarization is about an order of magnitude greater than for the pump polarization being orthogonal to a linear signal polarization. Thus the gain of a Raman amplifier in the field depends on the relative polarizations of the signal and the pump, which may fluctuate randomly. This results in uncertainty in amplifier performance, leading to increased errors in signal detection or increased system margin requirements.
Therefore, there is a need for pump laser systems for fiber amplifier systems that can produce a depolarized output while also delivering the desired range of pump wavelengths, so that amplifier performance can be more predictable and reliable.
In general, the present invention is directed to apparatus and method for depolarizing the output from two or more lasers. This is particularly advantageous for producing high power, depolarized light for pumping a fiber Raman amplifier, or for producing multiple wavelength, depolarized light for pumping a fiber Raman amplifier.
In one embodiment, the invention is directed to an optical device that includes a polarization combiner having a first input for light in a first polarization state and a second input for light in a second polarization state orthogonal to the first polarization state, and having an output for light mixed at the first and second polarization states. The device also includes a first polarization maintaining (PM) fiber having an input coupled to the output of the polarization combiner and defining orthogonal fiber polarization modes. The polarization maintaining fiber input is oriented so that light output from the polarization combiner in the first polarization state excites the orthogonal fiber polarization modes substantially equally.
In another embodiment, a laser system includes a first laser generating a first polarized output and a second laser generating a second polarized output. A polarization combiner has a first input coupled to receive the first polarized output and a second input coupled to receive the second polarized output polarized orthogonally to the first polarized output. The polarization combiner has an output for light comprising the first polarized output and the second polarized output. A first polarization maintaining fiber has an input coupled to the output of the polarization combiner, and defines orthogonal fiber polarization modes. The first polarization maintaining fiber input is oriented so that light output from the polarization combiner from each laser excites the orthogonal fiber polarization modes substantially equally.
Another embodiment of the invention includes polarization combining means for combining two light beams of first and second mutually orthogonal polarizations to a mixed polarization output, and depolarization means for depolarizing light coupled to receive the mixed polarization output from the polarization combining means, light of the mixed polarization output at each of the mutually orthogonal polarizations exciting polarization modes of the depolarization means equally.
Another embodiment of the invention is directed to a method of producing a depolarized light beam. The method includes combining first and second light beams of respective first and second mutually orthogonal polarizations to produce output light, and exciting orthogonal polarization modes of a polarization maintaining (PM) fiber substantially equally with the output light from the first light beam. The method also includes exciting orthogonal polarization modes of the PM fiber substantially equally with the output light from the second light beam.
Another embodiment of the invention is directed to a depolarizing device that includes a polarization combiner having a first input for light in a first polarization state and a second input for light in a second polarization state orthogonal to the first polarization state, and having an output for light mixed at the first and second polarization states. The depolarizing device also includes a depolarizer that defines first and second orthogonal polarization modes coupled to receive light from the polarization combiner output, the first and second polarization modes each being oriented so that the light output from the polarization combiner in the first polarization state is depolarized by the depolarizer independently of the light in the second polarization state and the light output from the polarization combiner in the second polarization state is depolarized by the depolarizer independently of the light in the first polarization state.
Another embodiment of the invention is directed to a method of producing a highly depolarized light beam. The method includes combining a first light beam of a first polarization state with a second light beam of a second polarization state orthogonal to the first polarization state to produce output light having a mixed polarization of the first and second polarization states. The method also includes orienting a single stage depolarizer so that output light in each of the first and second polarization states is depolarized by the depolarizer independently of the other polarization state, and passing the output light through the single stage depolarizer so as to depolarize the output light.
Another embodiment of the invention is a laser system that includes a first laser generating a first polarized output and a second laser generating a second polarized output. The laser system also includes a polarization combiner having a first input coupled to receive the first polarized output and a second input coupled to receive the second polarized output polarized orthogonally to the first polarized output, and having an output for light comprising the first polarized output and the second polarized output. The laser system further includes a depolarizer defining first and second orthogonal polarization modes coupled to receive light from the polarization combiner output, the first and second polarization modes each being oriented so that the light output from the polarization combiner in the first polarization state is depolarized by the depolarizer independently of the light in the second polarization state and the light output from the polarization combiner in the second polarization state is depolarized by the depolarizer independently of the light in the first polarization state.